1. Field of the Invention
The present invention relates to a sheet stacking apparatus that stacks sheets which have been conveyed.
2. Description of the Related Art
Conventionally, an image forming apparatus which has a sheet processing apparatus incorporated in a housing thereof has been proposed (Japanese Patent Application No. 2001-313041). An image forming apparatus of this type in which sheets are discharged inside the housing enables a large reduction in the space occupied by the sheet processing apparatus. FIG. 29 is a sectional view showing the construction of the conventional image forming apparatus of the type in which sheets are discharged inside the housing. In this image forming apparatus, a processing tray 1001 is mounted in an inclined fashion inside a sheet processing apparatus 1000. Sheets S on which images have been formed inside the main body of the image forming apparatus are temporarily stacked on the processing tray 1001 inside the sheet processing apparatus 1000, and the sheets are then subjected to post processing such as aligning and stapling on the processing tray 1001.
In the sheet processing apparatus 1000 in which the processing tray 1001 is mounted in an inclined fashion, it is intended that sheets discharged onto the processing tray 1001 slide along an inclined surface of the tray to move backward, and hence the sheets on the processing tray 1001 can be aligned in the sheet conveying direction by a simple return roller. Further, a sheet processing apparatus exists, in which a processing tray is mounted with a reduced angle of inclination thereof so that it is disposed on a substantially horizontal plane, in order to have an increased number of sheets stacked thereon. This sheet processing apparatus is capable of performing sheet discharge speed control such that sheets discharged from the image forming apparatus has a jumping amount thereof controlled to a substantially constant amount according to conditions of the sheets such as sheet size and sheet material.
Further, another sheet processing apparatus is known, which has a pair of discharging rollers composed of an upper roller and a lower roller provided at an sheet discharge outlet of a processing tray disposed substantially horizontally, to convey sheets discharged from the image forming apparatus (Japanese Laid-Open Patent Publication (Kokai) No. H11-171396). This sheet processing apparatus has a roller position changing means for displacing the upper roller selectively into an operating position in which the upper roller is placed in contact with the peripheral surface of the lower roller and into a receding position separate from the operating position, and a driving means for driving the upper roller to rotate forward or backward.
In this sheet processing apparatus, to convey sheets discharged from the image forming apparatus onto the processing tray, control is provided to set the upper roller into the operating position and drive the same to rotate backward. On the other hand, to convey a bundle of sheets subjected to post processing on the processing tray onto a stack tray, control is provided to set the upper roller into the operating position and drive the same to rotate forward.
Still another sheet processing apparatus is known, in which sheets with images formed thereon are conveyed and stacked on an intermediate stacker as a processing tray, followed by being subjected to post processing, and the post processed sheets are discharged onto and stacked on a stack tray (Japanese Laid-Open Patent Publication (Kokai) No. 2000-355455). This sheet processing apparatus is comprised of a pair of sheet discharging rollers composed of an upper sheet discharging roller and a lower sheet discharging roller which are rotatable forward and backward to sandwich or catch sheets on the intermediate stacker therebetween to convey and discharge them, a crimp means for swinging the upper sheet discharging roller selectively into urging contact with or separate from the lower sheet discharging roller, and a driving means for driving the sheet discharging rollers to selectively rotate forward or backward while activating the crimp means. In the sheet processing apparatus, the stack tray is disposed at a lateral side surface of the image forming apparatus in a manner inclined through approximately 30 degrees. A portion of the stacking surface of the stack tray at which the leading end of a sheet to be processed discharged onto the intermediate stacker by the sheet discharging rollers contacts the stacking surface of the stack tray is at a higher level than the highest portion of the intermediate stacker, and therefore the sheet to be processed discharged onto the intermediate stacker never drops down onto the stack tray due to its own gravity.
However, if the sheet processing apparatus is configured such that the stack tray is disposed at a location below an original reader and at an upper location inside the main body of the image forming apparatus, and with an angle of inclination smaller than that in the conventional configuration so as to secure a sufficient stacking capacity of the stack tray, the following problem arises. Therefore, it is difficult to obtain a sufficient stacking capacity of the stack tray when it is disposed at a location below the original reader and at an upper location inside the main body of the image forming apparatus.
That is, when the sheet processing apparatus conveys a sheet received from the image forming apparatus and discharges it onto the intermediate stacker through the stack tray, the sheet cannot move backward due to its own gravity on the substantially horizontal inclined surface of the stack tray and hence cannot drop onto the intermediate stacker. Therefore, if the discharging speed of the sheet is too high, the sheet can drop onto the stack tray without sliding down to the intermediate stacker. Consequently, the sheet cannot be properly sandwiched or caught between the upper and lower sheet discharging rollers by the upper sheet discharging roller being lowered toward the lower sheet discharging roller. On the other hand, if the discharging speed of the sheet is too low, the sheet discharging rollers which are relatively weak in driving force cannot completely discharge the sheet onto the intermediate stacker, so that the trailing end of the sheet remains on the sheet discharging rollers.
Further, when a bundle of sheets post processed on the processing tray is discharged in a bundle onto the stack tray while being sandwiched between the upper and lower rollers, if the timing in which the upper roller is lifted is delayed, there is a fear that the leading end of a succeeding sheet discharged onto the processing tray collides with the upper roller or a member that swings the upper roller.
Moreover, even if the discharging speed of sheets is controlled to a constant speed, the actual discharging speed can increase depending upon the material of an image-formed sheet, so that the sheet can jump excessively and cannot be properly sandwiched or caught.
In addition, to reliably sandwich sheets, the sheet stacking surface of the processing tray has to be large, leading to an increased size of the apparatus.